Total organic carbon (TOC) is a commonly sought property of a hydrocarbon-bearing subsurface formation. In recent years the level of interest in the measurement of this property has increased even further with the emergence of shale oil and shale gas exploration and production. The shale formations being explored are typically more complex than conventional reservoir formations and they pose many more challenges in their petrophysical studies and interpretations. Many of the standard measurement techniques commonly used in conventional formations, such as measuring the TOC, do not work in shale. Traditionally TOC is derived from sonic or density logs. Both of those methods involve either prior knowledge or an accurate estimate of the matrix properties before the data can be interpreted. Estimation of matrix properties is not trivial for shale formations due to the high variability of constituent minerals and the possible presence of trace minerals such as pyrite, for example.
Shale formations are highly laminated and their depositional histories and transformation processes generally vary. The lamination thickness is not constant, but rather may vary anywhere in the range of millimeters to meters. As a result, high resolution measurements with short spacing between the sampling points can be important for evaluating the shales and to ensure any decision on the quality and economic potential of the formation reflects the real system.
Laser induced pyrolysis (LIP) has been used to make certain formation evaluation measurements uphole, at the surface. For example, LIP has been applied to core samples. LIP may also be used on rock cuttings flushed to the surface while drilling. However, one generally has no idea of the depth within the well from which the cutting came. That is, during drilling operations pieces of rock are cut and brought to the surface by the circulating drilling fluid (mud). While the mud travels to the surface, it experiences turbulent flow, causing the cuttings to mix and their relative depth information to be lost. In relatively homogeneous formations, measurements at the surface may succeed. However, shale cuttings, with their associated variable laminations, should not be considered to be from a homogeneous formation. A LIP measurement on a cutting may provide a high resolution map of the lamination of that cutting, albeit with uncertain depth information, but the obtained lamination map is generally not representative of the lamination of the shale reservoir.
The organic shales (oil or gas shale) are made of an inorganic matrix within which organic patches may be distributed. The organic patches may contain kerogen, for example, which is a source of producible oil or shale, and the kerogen content and its maturity are primary measures of producible liquid and gas hydrocarbons in these formations. Thus, its measurement provides an important parameter to assess the economic value of a particular shale formation. The percentage of kerogen is reported as total organic carbon (TOC). Higher TOC in a formation is expected to correlate with a higher volume of generated hydrocarbons (assuming the same maturity). Kerogen in the pore space also acts as a reservoir by adsorbing gas molecules inside the nano-pores that are present in its structure (i.e., intra-kerogen). Thus, quantifying TOC is an initial process in evaluating any shale gas or shale oil reservoir.